The principle goal of this project is to define the cellular states and signals that control the cell types and synapses in the central nervous system. The work in this program is based on the identification of stem cells in the nervous system. These cells can be obtained from the developing and adult nervous system and they can generate the differentiated cells of the nervous system, neurons, astrocytes and oligodendrocytes. In the past year, in these studies on the fundamental biology of nervous system stem cells we have:[unreadable] [unreadable] 1. used a high resolution imaging system to define the lineage and responses of stem cells differentiating into neurons, astrocytes and oligodendrocytes. This imaging system was built here and is the only one in the world that allows stable long-term culture of differentiating cells. Using this system we have shown that tri-potent stem cells generate the major cell types in the brain by first differentiating into progenitors with the potential two generate two fates and these cells, in turn, give rise to progeny that are restricted to a single fate. We have now shown that extracellular signals that regulate fate choice do not act on the tri-potent stem cells but alter the fate of the bipotent cells. In the last year, our studies show that the events occuring in a restricted time window in a cell type that is only transiently present regulate fate choice. This study will form the basis of an intense effort to define the signals controlling fate choice.[unreadable] [unreadable] 2. identified fundamental cell growth pathways downstream of the Notch receptor that promote embryonic, fetal and adult stem cell expansion in vitro and in vivo. This positive pathway is regulated by negative signals. These positive and negative systems contain the core components of the system known to control cancer. The oncogenes in the positive pathway and the tumor suppressors in the inhibitory loop. This system is regulated by a third pathway containing the p53 and p21 stress response genes that are also critical in cancer. In the last year, we have developed conditions to assess how p53 and p21 are involved in the growth control mechanisms of embryonic and somatic stem cells. This study allows us to ask if the function of p53 in stem cell differentiation is the function that leads to mutation in almost every human cancer.[unreadable] [unreadable] 3. used an in vitro model to show that hippocampal neuronal survival is only transiently dependent on neurotrophins. Previous work from our group identifies a serine site on STAT3 as an integrator of survival signaling initiated by insulin, gp130 and Notch receptors in embryonic, fetal and adult stem cells. In the last year we have shown that when neural stem cells generate neurons, STAT3 signals are blocked and restored at the time their survival mechanisms become robust. These data show that STAT3 plays a critical role integrating survival signals as stem cells differentiate into functional neurons. This model system will be extended to ask if all neurons use the same core survival machinery.